Synthesis of 2,2‘-Biimidazolium-Based Ionic Liquids: Use as a New Reaction Medium and Ligand for Palladium-Catalyzed Suzuki Cross-Coupling Reactions

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Abstract
Neat reactions of 2,2‘-biimidazole with an excess of alkyl or polyfluoroalkyl iodides at 140 °C, followed by anion exchange with LiN(SO2CF3)2 or KPF6, gave the diquaternary salts 3a−k in >80% yields. However, by controlling the reaction stoichiometry, 2,2‘-biimidazole can also be monoquaternized with the same electrophiles at 100 °C under similar conditions. Subsequent metathesis reactions with LiN(SO2CF3)2 or KPF6 resulted in the ionic liquids 4a−m in high yields. Thermal properties were determined with a differential scanning calorimeter (DSC) and a thermogravimetric analyzer (TGA). Most of the monoquaternary salts are room-temperature ionic liquids. 1,3,1‘-Tributyl-2,2‘-biimidazolium hexafluorophosphate was demonstrated to be an excellent solvent and ligand for palladium-catalyzed Suzuki cross-coupling reactions. The catalytic ionic liquid system may be recycled at least 14 times without a significant decrease in catalytic performance.